Investigators are continuing to investigate the cause of a fire Tuesday night that charred a Church Street apartment building and left an 18-year-old in critical condition with severe burns.

Firefighters were dispatched to the building at about 7 p.m., after neighbors reported seeing flames and hearing an explosion. The 18-year-old and an unidentified female were already outside the building when emergency personnel arrived on scene and were ultimately sent to an out-of-the-area hospital for treatment.

But it was what firefighters found when battling the blaze that ultimately led to the fire getting out of control, burning much of the building and leaving eight to 10 residents homeless.

Humboldt Bay Firefighters had largely extinguished the blaze and were mopping up when they reported coming upon a device for extracting concentrated THC from marijuana -- which they mistook for a pipe bomb -- causing them to pull back out of the building until the county bomb squad could investigate.

Firefighters found the device inside a bedroom closet in the apartment where the fire originated, according to Humboldt Bay Fire Chief Ken Woods. But upon finding the device -- not knowing what it was or what else may have been in the apartment -- fire crews were forced to evacuate the building and continue fighting the fire from the outside, he said.

In the time it took for the Humboldt County Bomb Squad to arrive, the fire had reignited and spread to other parts of the building via the attic, Woods said.

The Humboldt County Bomb Squad was notified about the device at about 8 p.m. and spoke with a Eureka Police Department detective about obtaining a search warrant while en route to the scene, said Lt. Steve Knight of the Humboldt County Sheriff's Office. Bomb technicians arrived on scene at about 9:30 p.m. with the approved search warrant and ultimately used the robot to investigate the suspicious device.

The bulk of the damage to the building and its contents -- about $500,000 total -- is due to the amount of time firefighters had to wait before being able to fight the fire from inside the building, Woods said.

"We set up about a 70-foot perimeter where our people were not close, and we were shooting water in through openings in the windows and through the hole in the roof trying to keep the fire contained," he said, adding that it took about three hours for the bomb technician to remove the device. "But because (the fire) was in the attic, it spread through probably 65 to 70 percent of the building before we could get a handle on it."

EPD detective Todd Wilcox said it was necessary to get the search warrant before the bomb squad entered the apartment to ensure that anything found could be used as evidence in a potential prosecution. Additionally, Wilcox said, emergency personnel had no idea what they would find upon further investigation of the residence.

"You can't start taking short cuts when you don't know what you're looking at and what you're going to find," Wilcox said, adding that police could have found a dead body in the residence and suddenly been dealing with a homicide investigation.

Woods, who went in with the bomb technician to show him where the device was, said there were still active flames when the bomb squad arrived. The technician determined that the device posed no threat, removed it, and firefighters were able to resume battling the blaze. Woods said fire investigators don't think the hashish extractor is the source of the actual fire, adding that the device was still intact when the bomb technician removed it. Firefighters also found butane cylinders in the apartment, he said.

Volunteer personnel with the Eureka Police Department blocked off Summer Street and Pine Street during the bomb squad's investigation. Fire crews were on scene until about 3:15 a.m. Wednesday, Woods said.

Bomb technicians removed a device that consisted of a plastic pipe with a cap at each end, Knight said. Holes had been drilled into each of the caps. Knight said the device technicians removed is similar to an extractor device made out of a galvanized pipe recovered during a marijuana bust in Hydesville Tuesday morning. The extractor device looks very much like a pipe bomb, he said.

According to Knight, manufacturers trying to concentrate THC fill the extractor device with marijuana and force butane through it to remove the THC from the plant matter of the marijuana. The substance that drips out of the device is the concentrated THC, he said. Manufacturers, Knight said, often work in areas with poor ventilation, and the butane fumes can pose a hazard.

"Any ignition source can cause it to explode," Knight said. "These things are very dangerous."

According to Eureka Police Chief Murl Harpham, police officers continued to investigate the scene of the fire on Wednesday. Harpham said he didn't have the results of that investigation.

During the initial fire, apartment building residents reported hearing an explosion before seeing smoke and flames. One resident, Heather Smith, who lives in a unit that faces Pine Street, said she thought the water heater might have exploded. Woods said firefighters haven't been able to confirm that yet.

With all the butane in the apartment -- Wilcox said numerous canisters were found -- it's also possible the explosion was caused by the combustible fumes catching a spark, the detective said.

Woods said it's simply too early to tell what caused the blaze or if it had anything to do with the hashish making equipment reportedly found on scene.

"We're looking at all sorts of ignition sources, the water heater being one of them," he said, "but we haven't nailed them down yet."

Firefighters responded to the blaze on the 200 block of Church Street at 7:04 p.m. Tuesday. At that point, the fire was centered in a second story apartment. City Ambulance immediately transported two of the apartment's inhabitants, an 18-year-old male and an unidentified female, to St. Joseph Hospital, Woods said. He estimated that the 18-year-old had serious burns covering about 60 percent of his body, and the female suffered from inhalation burns. The two victims were transported by helicopter to a burn unit, Woods said.

According to the University of California Davis Medical Center, the 18-year-old is in critical condition.

In all, firefighters evacuated eight to 10 residents from the building Tuesday night.

Smith and her family were busy removing furniture and other belongings from her home the afternoon after the fire. The living room and front bedroom were largely undamaged, but firefighters were forced to knock a hole in the kitchen and back bedroom ceiling to attack the blaze, which spread via the attic. Plaster, glass and insulation material littered Smith's bed and the bedroom carpet sustained massive water damage. Smith, who had been living in the home for a year and a half, said her landlord told her it may be four to six months before she or the other residents can move back into their homes.

Despite the damage, Smith said firefighters had managed to save most of her furniture, her clothing and her daughter's belongings. Smith said she's currently staying with family.

"I had just decorated it, finally turning it into our home," she said, adding that she may move to Fortuna. "But everybody's fine. My daughter's almost three, and she's handling it like a champ."

Out of the eight to 10 residents that were forced to leave their home due to the fire, all but two women were able to find shelter through family or friends, said Cris Jones, deputy regional disaster director for the Humboldt County chapter of the American Red Cross. The Red Cross helped the two women by putting them up in a motel for two nights and giving them vouchers for clothing and food.

Jones said if any of the other residents have additional needs, they can call the Red Cross at 443-4521 or the community switchboard at 441-1001.

Woods said the cause of the fire remains under investigation and Wilcox said EPD is continuing to investigate the possibility there was a hash making operation in the residence, and may refer the case to the Humboldt County District Attorney's Office for possible criminal charges.